This invention relates to refining of glyceride oils by contacting the oils with an adsorbent capable of selectively removing trace contaminants. More specifically, it has been found that novel acid-treated silica supports have superior properties for the removal of chlorophyll and phospholipids from glyceride oils. This facilitates the production of oil products with substantially lowered concentrations of these trace contaminants. The term "glyceride oils" as used herein is intended to encompass all lipid compositions, including vegetable oils and animal fats and tallows. This term is primarily intended to describe the so-called edible oils, i.e., oils derived from fruits or seeds of plants and used chiefly in foodstuffs, but it is understood that oils whose end use is as non-edibles (i.e., technical grade oils) are to be included as well. It should be recognized that the method of this invention also can be used to treat fractionated streams derived from these sources.
Refining of crude glyceride oil purifies the oil of many undesirable substances, including color components (such as chlorophyll A and red and yellow color bodies), phospholipids, free fatty acids and other volatile species that impart undesirable colors, flavors and odors to the oil. Removal of these species results in oil having good appearance, flavor, odor and stability. Many of these species are removed by contacting the oil with an adsorbent (i.e., bleaching earths or amorphous silica). Various forms of acid treatment also have been used at different stages of the refining process, and for different purposes, in the oil refining industry.
Crude glyceride oils, particularly vegetable oils, are refined by a multi-stage process, the first step of which is degumming by treatment typically with water or with a chemical such as phosphoric acid, citric acid or acetic anhydride. For example, Vinyukova et al., "Hydration of Vegetable Oils by Solutions of Polarizing Compounds," Food and Feed Chem., Vol. 17-9, pp. 12-15 (1984), discloses degumming using a hydration agent containing citric acid, sodium chloride and sodium hydroxide in water to increase the removal of phospholipids from sunflower and soybean oils. U.S. Pat. No. 4,049,686 (Ringers et al.) discloses dispersing a substantially concentrated acid or anhydride in the oil, adding water and separating the aqueous phase containing gums and phospholipids. In addition to the use of organic acids during oil degumming, citric acid and other weak acids have been used as trace metal deactivating agents to promote taste and oxidative stability of edible oils.
After degumming, the oil may be refined either by a chemical process including neutralization, bleaching and deodorizing steps or a physical process may be used, including a pretreating and bleaching step and a steam refining and deodorizing step. The removal of phospholipids and chlorophyll from edible oils has been the object of a number of previously proposed physical and chemical process steps. Clays or bleaching earths most commonly have been used for removing phospholipids and color bodies from glyceride oils. These adsorbents may be used in their naturally occurring form or they may be acid-activated prior to use. U.S. Pat. No. 4,443,379 (Taylor et al.) describes the bleaching clays and acid-activation method commonly used for this purpose, noting that Fuller's earth and acid-treated sub-bentonites have an adsorptive capacity for color impurities in oils and that the acid-treated sub-bentonites have the highest adsorptive capacity.
It is also known that amorphous silicas may be used in the oil refining process. U.S. Pat. No. 4,629,588 (Welsh et al.) teaches the utility of amorphous silica adsorbents for the removal of trace contaminants, specifically phospholipids and associated metal ions, from glyceride oils, (Parker et al.) (U.S. Pat. No. 4,734,226), teaches the removal of these trace contaminants by adsorbing onto amorphous silica which has been treated with an organic acid, such as citric acid, tartaric acid, acetic acid or ascorbic acid. Direct color improvement of glyceride oils has not previously been associated with the use of silica adsorbents in the bleaching step, although treatment with silica does facilitate and improve the decolorization which takes place in subsequent deodorization.
In current refinery practice, chlorophyll is most efficiently removed from glyceride oils by the use of acid-activated clays. Although commonly used in the industry, clays and bleaching earths suffer from a number of disadvantages. They typically do not filter well and are associated with significant oil losses. Moreover, spent bleaching earth has a tendency to undergo spontaneous combustion, making its handling somewhat hazardous.